The present invention relates to probing assemblies of the type commonly used for testing integrated circuits (ICs) and, in particular, to a probing assembly providing finely pitched, compliant probes having very low inductance.
Integrated circuit technology permits fabrication of a number of discrete electronic circuit elements on a single substrate or “wafer.” After fabrication, this wafer is divided into a number of rectangular-shaped chips or dies where each die includes a rectangular or other regular arrangement of metallized contact pads or bond pads through which input and output connections can be made to the electronic circuit on the die. Although each die is eventually packaged separately, for efficiency, testing of the circuit formed on each die is preferably performed while the dies are still joined together on the wafer. One typical procedure is to support the wafer on a flat stage or “chuck” and move the wafer in X, Y, and Z directions relative to the head of a probing assembly so that probe tips projecting from the probing assembly can be moved from die to die for consecutive engagement with the contact pads of each die. Respective signal, power, and ground conductors connect the probe tips to test instrumentation enabling each circuit to be sequentially connected to and operated by the test instrumentation.
One type of probing assembly used for testing integrated circuits utilizes a plurality of needle-like contacts arranged in a pattern matching the pattern of the contact pads on the device to be tested. FIGS. 1 and 2 show a probing assembly 20 that includes a needle card probe head 22 comprising an array of needle-like probes 24 restrained by upper 26 and lower 28 needle cards. The upper and lower needle cards 26, 28 contain patterns of holes that correspond to the contact pad arrangement of the IC or other device to be tested with the probing assembly 20. The lower end of each of the probes 24 extends through one of the holes in the lower needle card 28, terminating in a pointed probe tip. The upper end of each of the probes 24 is restrained by a hole in the upper needle card 26. The holes of the upper needle card 26 are covered by electrically conductive pads 32 arranged on a surface of a space transformer 30 (indicated by a bracket) preventing the upper ends of the probes from sliding through the upper needle card 26 when the lower ends of the probes are brought into pressing engagement with the contact pads on the device under test. The space transformer is a rigid, multilayer plate having electrically conductive contacts 32, 36 on the opposing surfaces that are electrically connected by conductive traces 34 that extend through the plate. The space transformer 30 re-routes the electrical signals from the finely pitched pattern of the needle probes 24 to a more coarsely pitched pattern obtainable on a probe card 38, a printed circuit board through which the test instrumentation is connected to the probing assembly.
The exemplary probing assembly 20 also includes an interposer 39 disposed between the space transformer 30 and the probe card 38. The interposer 39 typically includes a plurality of elastically deformable contacts electrically connected through a substrate to provide compliant electrical connections on opposing sides of the substrate. The compliance of the conductors compensates for variations in the distances separating the respective terminals of the space transformer 30 and the probe card 38 promoting reliable electrical connections there between.
The needle probes 24 typically comprise a wire including complementary bends that form an upper section and a lower section that lie generally parallel to, but offset from each other, adjacent, respectively, the upper and lower ends of the probe. The hole pattern of the lower needle card 28 is offset from the hole pattern in the upper needle card 26 to accommodate the offset of the ends of the probes. When the lower end of a probe is pressed into engagement with the contact pads on a die, the substantially columnar probe can bend at the offset, acting like a spring. The compliance provided by the elastic bending of the probe accommodates variations in probe length, probe head planarity, and wafer topography.
Needle card probing assemblies have been used extensively in wafer testing, but the trend in electronic production, and, in particular, IC production, to higher frequency, more complex circuits having smaller circuit elements and geometries has exposed several limitations of this type of probing device. First, the pitch, the distance between the probes, is limited by manufacturing tolerances and assembly considerations to about 125 □m, a spacing greater than desirable for many ICs having finely pitched contact pads. In addition, the metallic contact pads of the dies oxidize rapidly and the tip of the probe must sharpened so that it can be pushed into the surface of the contact pad to achieve the good conductivity required for accurate measurements. This causes rapid dulling of the pointed probe ends, frequent bending or breaking of the probes, and may damage the contact pad if penetration is too great. The contact pad material also adheres to the probe and frequent cleaning is required which often damages the probes. Moreover, the inductance of parallel conductors is a function of the length and distance between the conductors. Typically, the relatively long, closely spaced, needle-like probes exhibit a single path inductance of 1-2 nH which is sufficient to substantially distort high frequency signals, limiting the usefulness of needle-type probes for testing high frequency devices.
A second type of probing assembly is described by Gleason et al. in U.S. Pat. No. 6,708,386 B2, incorporated herein by reference. Referring to FIG. 3, a membrane probing assembly 40 includes a probe card 52 on which data and signal lines 48, 50 from the instrumentation are arranged and a membrane probing assembly 42. Referring to FIGS. 3-4, the membrane probing assembly 42 includes a support element 54 formed of incompressible material such as a hard polymer. This element is detachably connected to the upper side of the probe card by screws 56 and corresponding nuts 58 (each screw passes through a respective attachment arm 60 of the support element, and a separate backing element 62 evenly distributes the clamping pressure of the screws over the entire back side of the supporting element). Different probing assemblies having different contact arrangements can be quickly substituted for each other as needed for probing devices having different arrangements of contact pads.
Referring to FIGS. 4-5, the support element 54 includes a rearward base portion 64 to which the attachment arms 60 are integrally joined. Also included on the support element 54 is a forward support or plunger 66 that projects outwardly from the flat base portion. This forward support has angled sides 68 that converge toward a flat support surface 70 so as to give the forward support the shape of a truncated pyramid. Referring also to FIG. 4, a flexible membrane assembly 72 is attached to the support after being aligned by means of alignment pins 74 included on the base portion. This flexible membrane assembly is formed by one or more plies of insulative polyimide film, and flexible conductive layers or strips are provided between or on these plies to form the data/signal lines 76.
When the support element 54 is mounted on the upper side of the probe card 52 as shown in FIG. 5, the forward support 66 protrudes through a central opening 78 in the probe card so as to present the contacts which are arranged on a central region 80 of the flexible membrane assembly in suitable position for pressing engagement with the contact pads of the die or other device under test. Referring to FIG. 4, the membrane assembly includes radially extending arm segments 82 that are separated by inwardly curving edges 84 that give the assembly the shape of a formed cross, and these segments extend in an inclined manner along the angled sides 68 thereby clearing any upright components surrounding the pads. A series of contact pads 86 terminate the data/signal lines 76 so that when the support element is mounted, these pads electrically engage corresponding termination pads provided on the upper side of the probe card so that the data/signal lines 48 on the probe card are electrically connected to the contacts on the central region.
The probing assembly 42 is capable of probing a dense arrangement of contact pads over a large number of contact cycles in a manner that provides generally reliable electrical connection between the contacts and pads in each cycle despite oxide buildup on the contact pads. The membrane assembly is so constructed and connected to the support element that the contacts on the membrane assembly wipe or scrub, in a locally controlled manner, laterally across the contact pads when brought into pressing engagement with these pads.
FIG. 8 is an enlarged view of the central region 80a of the membrane assembly 72a illustrating an embodiment in which the contacts 88 are arranged in a square-like pattern suitable for engagement with a corresponding square-like arrangement of contact pads on a die. The membrane assembly provides space transformation from the very fine pitch of the densely packed contacts 88 to the more coarsely pitched contact pads 86 terminating the data/signal lines 76.
Referring also to FIG. 9a, which represents a sectional view taken along lines 9a—9a in FIG. 8, each contact comprises a relatively thick rigid beam 90 at one end of which is formed a rigid contact bump 92. The contact bump includes thereon a contacting portion 93 which comprises a nub of rhodium fused to the contact bump. Using electroplating, each beam is formed in an overlapping connection with the end of a flexible conductive trace 76a to form a joint therewith. This conductive trace in conjunction with a back-plane conductive layer 94 effectively provides a controlled impedance data or signal line to the contact because its dimensions are established using a photolithographic process.
The membrane assembly is interconnected to the flat support surface 70 by an interposed elastomeric layer 98, which layer is coextensive with the support surface and can be formed by a silicone rubber compound. The flat support surface, as previously mentioned, is made of incompressible material and is preferably a hard dielectric such as polysulfone or glass. When one of the contacts 88 is brought into pressing engagement with a respective contact pad 100 of a die, as indicated in FIG. 10, the resulting off-center force on the rigid beam 90 and bump 92 structure causes the beam to pivot or tilt against the elastic recovery force provided by the elastomeric pad 98. This tilting motion is localized in the sense that a forward portion 102 of the beam moves a greater distance toward the flat support surface 70 than a rearward portion 104 of the same beam. The effect is such as to drive the contact into lateral scrubbing movement across the contact pad with a dashed-line and solid-line representation showing the beginning and ending positions, respectively, of the contact on the pad. In this fashion, the insulating oxide buildup on each contact pad is abraded so as to ensure adequate contact-to-pad electrical connections.
A locally scrubbing, membrane probing assembly provides contacts which can be finely pitched to engage contact pads on physically smaller devices and combines high conductivity with ruggedness and resistance to wear and damage. Membrane suspended probes can also combine a greater section and shorter length to exhibit much lower inductance than typical needle probes permitting their use at higher frequencies and producing less signal distortion at all frequencies. However, the probes and the signal and data lines are created on the surface of the membrane and connect to probe card terminals arranged around the periphery of the membrane. Heretofore, membrane suspended probes have not been adaptable for use with the probe cards and space transformers suitable for use with a needle card-type probe heads where the signal paths pass through the center of the probing assembly and are arranged substantially parallel to the central axis of the probing assembly. What is desired, therefore, is a device and method for adapting robust, finely pitched, low inductance membrane suspended probes for use with the components of a probing assembly suited for use with a needle-type probe head.